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chapter31
Endocrine Metabolism II: Hypothalamus and Pituitary
TABLE 31-8
Factors That Affect the Neuroendocrine System
Changes in circulating levels of:
ACTH
PRL
GH
TSH
LHt
ADH
Stress
ttt
t
tt
*
4
t
testosterone
Exercise
t
11
ttp 4 4
A
t
Sleep
t
ttt
*
t
Insulin hypoglycemia
*
t
tt
t
Mild starvation
t
11
Arginine i.v.
tt
Emotional depression
♦ ft
4«
4
4
response to TRH
l-DOPA
Ml
tt
f P = pubertal child; A = adult,
i.v. = intravenous.
♦ = increased;
4
= decreased.
in the release of adrenal medullary hormones, since both
unpleasant (e.g., painful) or pleasant (e.g., sexually arous-
ing) stresses, if intense enough, will cause elevation of cat-
echolamine release. Unlike that of ACTH, the epinephrine
response to stress appears not to be diminished by
adaptation.
Almost every process in the body is subject to daily
rhythms. If the rhythm occurs once a day, it is called
circadian; if it occurs once every 90-120 minutes, it is
called ultradian; and if it occurs once every hour, it is
called circhoral. Anterior pituitary hormones are released
in low-amplitude ultradian pulses, which, in the case of
LH and FSH, can become high-amplitude circhoral bursts
under conditions of intense stimulation. These patterns of
release reflect the pulsatile discharge of hypothalamic re-
leasing factors and are components of a large, circadian
rhythm. The daily peaks and troughs in circulating lev-
els of ACTH and TSH are authentically circadian; they
are coupled to the body temperature rhythm and oscil-
late in a predictable manner under normal conditions.
In contrast, GH and PRL do not undergo regular circa-
dian rhythmicity unless the individual maintains a regular
sleep schedule, since circadian release of GH and PRL is
sleep-induced, that of GH being stimulated by deep sleep
(EEG stages 3 and 4) and that of PRL being stimulated by
rapid eye movement (REM) sleep. For this reason, peak
GH release occurs early during sleep (when deep sleep
is prevalent), and PRL levels rise during the later periods
of sleep (when REM sleep dominates). In adult humans,
the level of LH shows no circadian variation, but during
puberty the release of LH is sleep-induced. Table 31-8
summarizes the circadian regulation of pituitary hormone
release.
Depressive states can alter significantly the basal rates of
pituitary hormone secretion and their circadian rhythms.
In fact, depression may be related to periods when hor-
monal rhythms are out of phase with other rhythms in the
body. Circulating levels of sleep-inducible hormones (GH,
PRL, LH) are lower in depressive states, whereas that of
ACTH is elevated. The basal level of ACTH in a depressed
individual is elevated to the extent that it flattens the circa-
dian oscillations of the hormone. These changes resemble
those seen during disruptive phase shifts (e.g., east-bound
trip, altered work schedule, etc.) and emphasize the impor-
tance of CNS influence on the release of anterior pituitary
hormones.
Supplemental Readings and References
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1597 (1997).
M. E. Freeman, B. Kanyicska, A. Lerant, and others: Prolactin: structure,
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P h ysio lo g ica l R eview s
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(2000).
S. E. Inzucchi and R. J. Robbins: Effects of GH on human bone biology.
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R. S. Jackson, J. W. M. Creemers, S. Ohagi, et al.: Obesity and impaired pro-
hormone processing associated with mutations in the human prohormone
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